The present invention relates to apparatus for controlling heating of fluids, and in particular, relates to apparatus for controlling and/or monitoring temperature of fluids being heated.
Many fluid heating vessels, such as kettles, urns or jugs, utilize electronic controls to attain and maintain the fluid in the vessel at or near a predetermined temperature.
The basic vessel is generally of an insulating material such as thermoplastic, thermoset plastic phenolic or porcelain china (such as old style jugs). Typically, the wall thickness is about 2-5 mm.
In most heating units for the vessels, the energy used is electrical, and the vessel incorporates a sheathed mineral insulated element, direct (wire in water) element, dielectric (2 plate) unit, for example. Any other fuel (gas/oil) is also sometimes used.
In the case of a sheathed electrical element a control circuit is used in the vessel. Typically, where electronic controls are used, the actual sensor is a temperature dependent resistor (or thermistor). In this case the thermistor changes its resistance in accordance with changes in the temperature around the thermistor. The output of the control is in response to the changes sensed by the thermistor either switched on or off (proportional control) or the power supplied to heat the fluid is changed using wave chopping phase modulation, or other known techniques to increase or decrease the amount of power supplied to the heating element as required.
FIG. 1 illustrates some typical problems encountered in vessels used for the heating of fluids, for example water. Path 1 represents the temperature of fluid in a vessel when a great deal of energy is used to heat the fluid quickly. The fluid temperature rises in a short period of time and when the fluid temperature approaches boiling point (100.degree. C.) and just before point A, power is switched off from the heating element. However, the fluid temperature undesirably continues to increase into "boil over", between points A and B, before reducing to reach the desired fluid temperature (say 98.degree. C.). In the case of an urn or large fluid heating vessel, this temperature must then be maintained. Energy is wasted between points A and B.
Path 2 illustrates an alternative, where the fluid is slowly heated over a long period of time. This avoids "boil over", however, it takes far too long for the fluid to reach its desired temperature.
The problems illustrated in FIG. 1 principally stem from:
(1) Time constant of the fluid temperature monitoring device (thermistor); PA1 (2) Thermal "lag" due to indirect sensing; PA1 (3) Ambient temperature changes; and PA1 (4) Proximity of desired temperature to boiling point of the fluid, for example water (100.degree. C.). PA1 (1) In fluid vessels, with a thermistor located in the fluid (direct sensing), the reaction time of the thermistor itself, on fast heating units, can cause "overshoot" in simple controls as shown in FIG. 1, path 1. Also there exists technical problems of electrically isolating the thermistor leads, as well as preventing corrosion thereof. This generally leads to a design that to all intents, looks similar to indirect designs as described below. PA1 (2) As shown in FIG. 2, a typical indirect sensing application is shown in a water heating urn 3. The water temperature sensing thermistor 4 is pushed into a preformed pocket 5 either provided in the water heating chamber wall 6 or which is integrally molded with the vessel wall. Generally, a heat conducting paste 7 is used to enhance the sensing of the water 8 temperature by the thermistor 4. Thermal lag occurs due to the time required for a temperature change in the fluid to be transmitted through the vessel wall 6, the paste 7 and the body of the thermistor 4 before being (electronically) sensed by the thermistor. PA1 (3) An ambient thermistor 9 actually reads a temperature somewhere between the temperature of the water 8 and the temperature of the air 11 which exists between the water heating chamber 6 and the outside wall 10 of the urn 3. A printed circuit board is mounted to carry ambient temperature thermistor 9 and the water temperature thermistor 4. From cold start up, the temperature around the PCB will slowly rise from room ambient temperature to steady state operating temperature, providing delays in heating the fluid as desired. PA1 (4) Water at normal atmospheric pressure boils at 100.degree. C. However, at or above 100.degree. C. any energy added is dissipated as steam as no temperature increase is possible. If water temperature is desired at say 97.degree. C., and if the heating rate is faster than the thermistor/thermal lag etc. can cope with, then the water temperature will pass through 97.degree. C., through boiling point, and the fluid will boil until the thermistor catches up. This could be, typically, half a minute or more as shown in FIG. 1, path 1. This is due to the fixed "set point" by which the heater element control is influenced. The "set point" is the temperature sensed at which the heating control circuit turns off or reduces heating of the fluid, during fluid heating to a desired level. If the vessel heating control is an on/off type, then the full element wattage is applied to the water after boiling; this is undesirable. PA1 fluid temperature sensing means adapted to provide an indication of the temperature of fluid in said vessel; PA1 control means adapted to regulate the supply of heat to said fluid during a heating action; and PA1 ambient temperature sensing means for providing an indication of ambient temperature and for generating a control signal indicative of the sensed temperature, said ambient means being coupled to said control means wherein the magnitude of the control signal is varied in accordance with the temperature sensed by said ambient means to substantially prevent the temperature of said fluid rising to an undesirable level. PA1 fluid temperature sensing means adapted to provide an indication of the temperature of fluid in said vessel, PA1 control means coupled to said fluid means and adapted to regulate the supply of heat to said fluid during a heating action in accordance with a set point; and PA1 set point means adapted to provide an adjustable set point, said set point means being influenced by sensed ambient temperature.
These problems appear to result for the following reasons: